Messages in this thread | | | Date | Mon, 01 Mar 2010 15:15:53 -0500 | From | Jeff Garzik <> | Subject | Re: [RFC 1/4] libata: cache device select |
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On 02/17/2010 08:10 AM, Alan Cox wrote: > Avoid the device select overhead on every qc_issue (> 10uS) by caching the > currently selected device. This shows up on profiles under load. Best case > this costs us 10uS for the delay, worst case with a dumb interface it's > costing us about *1mS* a command. > > I believe the logic here is sufficient, but would welcome some second reviews > as its not something you want to get wrong ! > > > Signed-off-by: Alan Cox<alan@linux.intel.com> > --- > > drivers/ata/libata-sff.c | 8 ++++++-- > include/linux/libata.h | 1 + > 2 files changed, 7 insertions(+), 2 deletions(-) > > > diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c > index 63d9c6a..cf0332a 100644 > --- a/drivers/ata/libata-sff.c > +++ b/drivers/ata/libata-sff.c > @@ -469,6 +469,7 @@ void ata_sff_dev_select(struct ata_port *ap, unsigned int device) > > iowrite8(tmp, ap->ioaddr.device_addr); > ata_sff_pause(ap); /* needed; also flushes, for mmio */ > + ap->sff_selected = device; > } > EXPORT_SYMBOL_GPL(ata_sff_dev_select); > > @@ -1538,7 +1539,8 @@ unsigned int ata_sff_qc_issue(struct ata_queued_cmd *qc) > } > > /* select the device */ > - ata_dev_select(ap, qc->dev->devno, 1, 0); > + if (qc->dev->devno != ap->sff_selected) > + ata_dev_select(ap, qc->dev->devno, 1, 0); > > /* start the command */ > switch (qc->tf.protocol) {
My main worry here is that this logic excises the 150ms wait in ata_dev_select() that has been used effectively to allow ATAPI devices to "collect themselves" after waiting for idle, prior to command issuance.
Jeff
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